1. Field of the Invention
The present invention generally relates to a method for producing foil electrode for use in aluminum electrolytic capacitor. Particularly, it is concerned with a method for etching an aluminum foil which has an excellent characteristic as an electrode.
2. Description of the Prior Art
As is well known, the aluminum foil employed as an electrode for electrolytic capacitor must have a coarse surface of an extended effective surface area for the purpose of miniaturizing the size of, minimizing manufacturing cost of and/or improving the electrical characteristics of the products. Treatments of the aluminum foil for making it to have the coarse surface of the extended effective surface area are usually referred to as "etching process" and have customarily been performed in an aqueous solution by chemically or electrochemically.
In this field of etching process, there has hitherto been made a variety of studies and a number of proposals for attaining great industrial advantages. As a result of the detailed analysis of these studies, the following four principal factors, which greatly influence the performance of the etching process, i.e., extending rate of the effective surface area of the etched foil, are pointed out, and individual technical know-hows are established for each of the four factors:
1. Aluminum foil (purity of the metal; species, amount and mode of dispersion of impurities in the metal; orientation, size and shape of the crystal grain of the metal; and oxide film covering the surface, state and thickness),
2. Etching solution (species and concentrations of principal ingredient and additives; and temperature),
3. Power supply (direct current, alternating current, direct current superimposed with alternating current, waveform; and current density), and
4. Auxiliary treatment (pretreatments, intermediate treatments and combinations of these treatments).
Although it has been known that all of these factors can separately and independently influence the performance of the etching process and the configuration of the etched surface of the aluminum foil, it is confirmed that what is highly important for the performance and the configuration is the complexities, i.e., the interactions or mutual dependencies between and among these individual factors. In the other words, one of these factors in a given state may adversely interfere or synergetically cooperate with the other factor to create a combined influence on the performance of the configuration.